The invention relates to flow improving agents for binder suspensions containing cement based on sulfite-modified amino-s-triazine-formaldehyde condensation products and casein and it also relates to their use for producing self-levelling smoothing compositions.
The use of sulfite-modified amino-s-triazine-formaldehyde condensation products as flow improving agents in the processing of dry mortar mixtures, pourable mortars and other cement-bonded building materials is known.
Melamine is mainly used in this connection as the amino-s-triazine which is why these flow improving agents are known as MFS resins in the trade. They cause a strong liquefaction of the mortar mixture without undesired side effects occurring in the processing or in the functional properties of the hardened building material.
It is well known that commercially available flow improving agents based on melamine-formaldehyde-sulfite (e.g. Melment F 10.RTM. from the SKW Trostberg AG Company) produce an excellent liquefying effect even at very low dosages of about 0.3 to 1.2% by weight relative to the weight of cement.
The liquefying effect is achieved without lowering the surface tension of the water in the binder system which is usually the case for example with sulfonated naphthalene-formaldehyde condensation products (NFS resins) or flow improving agents with a surfactant-like polymer structure. The advantage of MFS resins is presumed to be due to the fact that no air voids are introduced into the mortar during the mixing process and the mortar density and strength are not adversely affected after hardening.
In addition MFS resins provide the fresh mortar mixture with a good cohesive strength so that even when the flow properties are extreme separation phenomena do not occur.
This phenomenon also called segregation, is feared especially in the production of self-flowing smoothing compositions (e.g. in the case of self-levelling screeds) since it leads to a non-uniform layer structure of the screed due to floating of the fine material and sedimentation of the coarse grain.
The result is strains within the material with a risk of stress cracking as well as an unfavourable time-course of drying due to the formation of a particularly dense and thus water-impermeable layer of fine material at the surface.
However, a disadvantage of MFS flow improving agents is that, despite the high initial efficacy, the flowing effect decreases relatively rapidly. Thus when used in smoothing compositions undesired variations in the flow property of the mortar occur within the technically applicable time periods.
In contrast it is known that casein as a mortar additive causes the so-called "self-healing" phenomenon.
"Self-healing" is understood to mean that a mortar to which casein has been added is still capable of smoothing out surface faults or flaws of cast mortar caused by processing which may be present even after a very long standing period by means of a very slow interflowing before hardening.
This "self-healing" can at present only be achieved with casein but not with conventional flow improving agents.
Therefore there have been many attempts to synergistically combine the properties of flow improving agents based on melamine with those of casein in self-levelling binder suspensions.
However, in doing so it has turned out that flow improving agents based on known amino-s-triazine-formaldehyde-sulfites cannot be combined with casein and that the simultaneous use of these resins and casein leads to a deterioration of the flow properties compared to the single mixtures.
The object of the present invention is therefore to develop flow improving agents which do not have the said disadvantages of the state of the art but which instead can be used in self-levelling smoothing compositions with good technical application properties.